The present invention relates to semiconductor lasers, and more particularly, to lasers that are coupled to light modulators.
Communication systems based on modulated light sources are well known to the art. In high-speed communication systems, the light source is typically a laser. At frequencies below 10 GHz, the modulation can be imparted to the light source by turning the laser on and off. Unfortunately, this type of laser modulation leads to increased line width in the laser light. At frequencies at or above approximately 10 Ghz, this increased line width cannot be tolerated.
Accordingly, light sources that are to be modulated at frequencies above 10 GHz are typically constructed by providing a laser that runs continuously and a separate light modulator that modulates the intensity of the laser output. The modulator typically has a transmissive state and an opaque state, which are switched back and forth by applying a potential across the modulator. In one type of light source, the laser and modulator are constructed on a common substrate. The length of the modulator section is preferably as short as possible to maximize the performance of the modulator. However, the length of the modulator must be sufficient to provide the desired contrast between the transmissive and opaque states of the modulator.
Broadly, it is the object of the present invention to provide an improved light source and modulator.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.
The present invention is a modulated light emitter having a laser and modulator constructed on a common substrate. The light emitter includes a first contact layer of a first semiconductor type and a first cladding layer on the contact layer of the first semiconductor type. An active layer is constructed on the cladding layer. The active layer includes a quantum well (QW) layer sandwiched between first and second barrier layers. A second cladding layer of the opposite semiconductor type and a second contact layer of the opposite semiconductor type are constructed over the active layer. The active layer includes a laser region and a modulator region connected by a waveguide. The laser region emits light when a potential is applied across the active layer in the laser region. The modulator region has a first state in which the modulator region absorbs the generated light and a second state in which the modulator region transmits the generated light. The modulator region assumes either the first or second state depending on the potential across the modulator region. The QW layer in the modulator region is under a tensile strain, which provides improved light absorption in the first state.